Paper is made by depositing cellulose fibers from a very low consistency aqueous suspension onto a relatively fine woven synthetic screen known as a forming wire or a forming fabric. A forming wire is a cloth woven from monofilaments, made endless by a seam to form a continuous belt. Both single and multi-layer wires are used in papermaking processes. The mesh of the wire permits the drainage of water while retaining the fibers. Over 95% of the water is removed by drainage through the forming wire.
Sheet formation on the forming wire is a complicated process that is achieved by three basic hydrodynamic processes: drainage, oriented shear and turbulence. The hydrodynamic effects must be applied in different degrees to optimize sheet quality for each grade of paper run on a paper machine.
There are many additives and processing aids that are used in a pulp and paper mill system. The addition starts with the incoming water and the wood chips going to the digester. Contaminants can also enter the system at this time. In fact, any additive to a pulp and paper system can introduce components that can end up as contaminants in a paper machine stock system. Contaminants and additives can build on the surface or become trapped between the multi-layer construction of the forming wire. High pressure water showers and low pressure chemical cleaning showers are used to remove deposits after the wet sheet leaves the wire. Any deposit on the wire can disrupt the sheet formation process by interfering with one or more of the three basic hydrodynamic processes.
After the formation of the wet paper web in the forming section of the paper machine, it is transferred to the press section by way of a pick-up roll. The primary purpose of the press section is to remove the maximum amount of water from the sheet before it enters the dryer section. The wet sheet will enter the press section at about 80% moisture and exit at approximately 55%. Maximizing moisture removal in the presses reduces the cost of operating the drying section. The press section can also improve properties such as sheet bulkiness and smoothness.
The press section removes water by running the sheet through a series of nip presses. A typical paper machine with a center roll will have three presses, each having two rolls and two wet press felts. As the wet web passes through a press, water removal is accomplished by squeezing the sheet through the nip of the two rolls. The two wet press felts (top and bottom) convey and support the wet sheet as it passes through the press and receives water expressed from the wet sheet in the nip.
Felt filling or plugging is caused by soils and additives becoming imbedded in the felt body thereby reducing the void volume and permeability, and in turn reducing the felt's ability to receive the water expressed from the web in the press nip. Almost all types of paper being recycled as broke contain a wide variety of potential system contaminants. For example, inorganic contaminants such as manganese, iron, copper and aluminum can deposit in wet press felts and on forming wires, thereby reducing drainage and causing runnability problems for the mill. High concentrations of mineral acids such as sulfuric acid-based cleaning compounds are usually required to remove the deposits. However, at times, the deposits can be so severe that they cannot be effectively removed with a full strength mineral acid compound. Moreover, high concentrations of mineral acids can severely damage press felts and forming wires.
Different processes and equipment are used to handle the complex challenge of separating useful fibers from inorganic and polymeric contaminants. However, regardless of how well this separation is accomplished, many microscopic and larger particles escape into accept streams and end up in the paper machine system. These particles lead to contamination of the paper machine felts. One such particle type is polyamide wet-strength resin associated with the manufacture of toweling grade tissues and other wet strength grades.
Over a period of time, resins can build in the void areas of the wet press fell and lead to reductions in permeability, as well as the ability of the felt to remove water. Currently, some mills will batch clean the felts with sodium hypochlorite. The major disadvantage of using sodium hypochlorite, however, is the degrading effect it can have on the nylon batt fibers. When the concentration of sodium hypochlorite exceeds 1 ppm for extended periods of time, it can cause premature felt damage. Moreover, production typically needs to be stopped to batch clean the felts with sodium hypochlorite, thereby leading to costly downtime.
In addition to the more traditional soils, spores and spore-forming bacteria can also accumulate in the felts. This can lead to a re-deposition of spores in the food grade board that increases the final spore count. If the spore count becomes too high, the board must be downgraded and sold in a non-food grade market. Sheath material associated with filamentous bacteria can also accumulate in the void area of the felt, thus resulting in a reduction in its ability to remove water. The problems associated with the buildup of sheath material can be experienced in any type of paper mill.
Accordingly, it would be desirable to provide an improved method of removing and preventing the buildup of contaminants in papermaking wet press felts and on forming wires without severely damaging the felts and wires. In particular, it would be highly desirable to utilize a cleaning solution to remove and prevent the buildup of manganese contaminants in wet press felts and on forming wires, as well as to remove and prevent the buildup of wet-strength resins, spores and sheath material from wet press felts during a normal continuous cleaning operation.